1. Field of the Invention
The present invention relates generally to a method of developing the contours of forming tools for members exhibiting complex shapes. The techniques of the present invention represent an improvement over those disclosed in commonly assigned U.S. patents, namely, U.S. Pat. No. 5,168,169 of H. Brewer and M. Holman entitled "Method of Tool Development" and U.S. Pat. No. 5,341,303 of S. Foroudastan and M. Holman entitled "Method of Developing Complex Tool Shapes". In this specific instance, the invention is directed to a methodology for simplifying the analysis of integrally stiffened structures of complex shape. While the instant disclosure refers to application of the technique of the invention on aluminum alloy material and also utilizes the principles of age forming for forming the member being fabricated, the invention need not be so limited. Indeed, the technique of the invention can be applied to any material for which there is a relationship between a strain in a member applied by a forming operation and a resulting strain in the member after the applied strain has been released. Thus, the invention can be applied to both cold forming and hot forming operations.
2. Description of the Prior Art
The complex shapes of the contoured members that make up aerospace structures are inherently difficult to form. Due to the shapes required by aerodynamics and because of the emphasis on load carrying capability combined with weight efficiency, optimized designs are created that require complex contours to be produced in high strength metals. Examples of such contoured members would include wing skin panels, fuselage panels, and structural stiffening elements such as spars and stringers for aircraft applications; as well as the shroud, skirt, and tankage members of space launch vehicles. Such members are characterized by extreme metal thickness variations and integrally machined features. The criticality of design requires precise forming tolerances be maintained without sacrificing the fatigue life, reliability, or strength of the member as a result of the forming process chosen.
Conventional forming methods, such as roll forming, brake forming, stretch forming, and peening, are cold working processes that achieve permanent deformations through the application of mechanical bending and/or stretching. Achieving uniform forming across integrally machined features or abrupt changes in thickness may not be possible without specialized tooling or extensive modifications to the forming equipment. In some cases, it may not be possible to develop the deformation forces necessary to accommodate extreme material thicknesses.
While various machines can handle a wide range of metal thicknesses, it is not practical to form metals varying from one extreme of the thickness range to the other, since most machines must be set up prior to operating. From this standpoint, skin tapers and recesses that occur within a panel may not be formable. Forming applications that have openings or cutouts machined into them may not be formable without distorting the opening or leaving flat spots in the contour. Other processes are limited by the size of the forming machinery and those applications that will fit within the working envelope. Custom equipment for larger or smaller applications can be prohibitively costly and inflexible.
In addition to the physical limitations imposed by part geometry are characteristic traits that result from the forming process used. Traits such as strain hardening, residual stresses, and marking accompany many of the forming processes commonly employed. In some cases these effects can produce desirable qualities, such as stress corrosion cracking resistance. Likewise others can produce undesirable qualities, such as a negative effect on the fatigue life and reliability of the formed part. The point to be made is that each forming process must be carefully matched to the intended application.
All of the conventional forming processes mentioned have one important disadvantage in common: each requires the expertise of a skilled operator. With the exception of some processes which have been automated to an extent, considerable operator skill is required to obtain tight tolerances; therefore, process consistency is low. Part to part variations in contour can result in engineering specified contour rework being required on every unit produced. Contour variations that do not require post forming corrections can still cause fit-up problems at assembly. Contour variations from part to part create numerous manufacturing difficulties, each with costly solutions.
While conventional cold forming processes have their drawbacks, they also have significant advantages for certain applications and tend to be much more economical than other known processes. It is noteworthy that the present invention can be applied to cold forming processes whenever it is practical to do so.
In the recent past, a significant advancement of known techniques for forming complex members while maintaining or even improving upon their inherent strength characteristics has been devised. Known as age forming, it is a process that offers many solutions to the problems encountered when conventional cold forming processes are applied to complex shaped contoured members. During age forming, a part is restrained to a predetermined tooling contour and precipitation aged. Age forming is a thermal forming process that utilizes the phenomenon of metallurgical stress relaxation during precipitation heat treatment for the purpose of converting elastic strain to a plastic state.
The age forming process may be performed on any of the precipitation heat treatable metals and metal alloys such as aluminum alloys in the 2xxx, 6xxx, 7xxx, and 8xxx series.
Age forming may be performed according to standard heat treatment cycles utilized in precipitation hardening of alloys. The underlying principles of precipitation heat treating are explained in "Aluminum Properties and Physical Metallurgy", Edited by John E. Hatch, American Society for Metals, Metals Park, Ohio, 1984, pp. 134-138 and 177-188, which is incorporated herein in its entirety by reference. As a result, suitable applications require the final condition of the formed components to be in an artificially aged temper. Every end use of a structure must be reviewed in light of the property changes that occur as a result of artificial aging. In some cases, the mechanical properties associated with an artificially aged temper may not be suitable for an intended application. As an example, aluminum alloy 2024 loses fracture toughness as it is artificially aged from the T3 to the T8 temper. This change presents a barrier to age forming applications where fracture toughness is a key design element, such as lower wing skins and fuselage panels for aircraft. Material and/or design changes are required in these cases to allow for the utilization of age forming. In other cases, age forming allows the added benefit of being able to produce contours in a strengthened temper, without developing high levels of residual stress within the component. An example of this feature is provided when aluminum alloy 7150 is age formed from the soft W temper to the hardened T6 temper.
More recently, the conventional age forming process has been modified and substantially improved through the use of the autoclave. The autoclave is a computer controlled pressure vessel, with the added benefit of being a certifiable source for heat treating aluminum. Age forming has traditionally been performed in a furnace, where a mechanical means of constraining the part to the predetermined forming shape is required. The autoclave offers the advantage of using vacuum and internal pressure to obtain the desired contour. Since pressure acts uniformly about the surface of the part, integrally machined features receive the same deformation force as the rest of the panel. Another important advantage is that the forming pressure is distributed about the entire surface area of the part. Therefore, a small differential pressure can equate to many tons of applied force when acting over a large surface. Most conventional processes concentrate the forming forces over a small area, thereby restricting the total available deformation force.
The autoclave is computer controlled allowing high levels of process consistency and accuracy. Computer control allows the process to be operator independent. A separate computerized system closely monitors and records the pressure and temperature within the autoclave providing traceability and process verification. These two features inherently endow autoclave age forming with high levels of process consistency and accuracy. Each panel receives the same processing; consequently, repeatability is ensured. It is this feature that makes the process adjustable. The tooling contour is "fine tuned" until the desired results are obtained.
Tooling for the autoclave is designed according to the springback anticipated for the application. Springback refers to the tendency for a member being formed to return to some shape intermediate its original shape and that of the tool to which it is subjected during heat treatment.
This phenomenon will be discussed at length below. Forming tools are designed with removable contour boards and other features that allow for rapid contour modifications. Unlike other forming processes, age forming does not typically allow for multiple forming iterations to be performed upon the same piece. Age forming is a heat treatment process; therefore, running a part more than once could result in over aging the material. Until the tooling contour is finalized, contour corrections must be performed by another forming process. Once the final tool contour is reached, secondary corrective forming processes are not necessary.
This inability to repeat the heat treatment process on a member being fabricated requires that it be scrapped if it exhibits an incorrect final contour and the procedure repeated with a new member. The cost of labor and materials for such necessarily repeated iterations of the process have led to the methods of the present invention.